Power amplifiers play an important role in both radio transmission and reception. For example, to produce an intelligible signal from radio transmissions received at an antenna, radio receivers must provide a significant amount of amplification. Similarly, radio transmitters will amplify their RF signals prior to transmission. In mobile applications such as wireless handsets, the power amplifier places the primary demand on battery charge. Thus, the efficiency of the power amplifier directly correlates with longer battery operation.
For efficient operation, a power amplifier's impedance should match the impedance of its antenna. If the impedance is not matched, a reflected wave will exist on the transmission line coupling the power amplifier and the antenna, lowering power efficiency. In addition, the impedance mismatch leads to frequency modulation distortion and a reactive component to the transmission line that may “frequency-pull” the power amplifier from its nominal frequency operating value. Accordingly, matching networks are used to couple between the power amplifier and antenna to provide impedance matching. However, depending upon the power output, a power amplifier's output impedance may change, making the design of such matching networks problematic. Moreover, a number of different types of matching circuits may be used such as inductor-capacitor (LC) networks, transformers, or transmission-line transformers. For example, U.S. Pat. No. 5,060,294 discloses a dual mode power amplifier that couples to its antenna through an LC network. Depending upon the mode of the power amplifier, the characteristics of the LC network change. Although the impedance of this matching LC network will change depending upon the mode, such networks are expensive and difficult to manufacture, especially at higher frequencies.
Moreover, at the higher frequencies such as used in the PCS band, an LC matching network may prove to be lossy compared to the use of a transmission line transformer. FIG. 1 illustrates a 4:1 Ruthroff transmission line transformer 5. Input port 7 is the low impedance side and output port 9 is the high impedance side. Thus, if a power amplifier having an output impedance of 12.5 Ω couples to input port 7 and a 50 Ω load couples to output port 9, the 4:1 impedance transformation provided by transmission line transformer 5 will match the source to the load. Unlike a conventional transformer, energy is not transmitted through transmission line transformer 5 solely by magnetic flux coupling. Instead, as the name implies, transmission line transformer 5 transmits energy through a transmission line mode. See, e.g, Jerry Sevick, Transmission Line Transformers, Noble Publishing Corp., 1996. The result is a broad-band impedance matching at low loss. However, this impedance matching will be constant and will not change in response to changes in power level or modulation mode.
Accordingly, there is a need in the art for improved matching networks that will adapt to various load or power conditions.